1. Field of the Invention
The invention relates to an inductive proximity switch with adjustable switching hysteresis, having a presence indicator including an oscillator with an oscillating circuit and an oscillator amplifier, a switch amplifier connected downstream of the presence indicator, an electronic switch controllable by the presence indicator through the switch amplifier, and a demodulator assigned to the oscillator.
2. Description of the Related Art
Such inductive proximity switches are known in principle and are described, among other sources, in German Published, Non-Prosecuted Application DE 37 22 335 A1, and in the publications mentioned therein.
In the known inductive proximity switches, the switch spacing is dependent on the quality of the resonant circuit of the oscillator. The quality of a resonant circuit, which includes a coil with a ferrite core and a capacitor, depends on the coil loss factor, among other factors. As can be found in the Siemens Data Book for 1990/1991, entitled "Ferrite und Zubehor" [Ferrites and Accessories], especially page 35, the coil loss factor is temperature-dependent. For the Siemens material known as N48 that can be used in inductive proximity switches, the loss factor is at a minimum, within the operating temperature range of interest for inductive proximity switches. In other words, above and below a certain temperature, the loss factor increases. In known inductive proximity switches, the temperature dependency of the switch spacing is partly compensated for with additional components, such as hot conductors, cold conductors, diodes or resistors. At least with a view to the simplest possible circuit layout using monolithic integration, such compensatory provisions are complicated, and the improvements in temperature dependency that are attained are relatively slight. With such known provisions, a deviation in the switch spacing of approximately .+-.10% over a temperature range of approximately 100.degree. can be attained.
It is accordingly an object of the invention to provide an inductive proximity switch with slight temperature dependency, with adjustable hysteresis and a presence indicator including an oscillator, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which has a switch spacing that is considerably less temperature-dependent, and which can be largely monolithically integrated.